An externally-mounted speed changing device, or rear derailleur, is mounted onto a bicycle, especially onto a sports-type road racer or mountain bike. Such rear derailleur is equipped with a base component, which is mounted on the frame; a link mechanism, one end of which is mounted on the base component; a movable component, which is mounted on the other end of the link mechanism, and which can be relatively displaced from the aforementioned base component; and a chain guide, which is mounted on the movable component so as to undulate like a see-saw. The chain guide functions to transfer the chain, so as to derail the chain upon any of the rear sprockets. The chain guide is equipped with an outer plate, which is mounted on the movable component so as to undulate like a see-saw; an inner plate, which is installed so as to face the outer plate; a guide pulley, which is mounted onto a portion on one edge of the plates, between the plates, so as to freely rotate, and which can be engaged by means of the chain; and a tension pulley, which can be engaged with the aforementioned chain, and which is mounted onto a portion on the other edge of the aforementioned plates, between the plates, so as to freely rotate.
With respect to the rear derailleur structured according to the above-stated manner, a rear derailleur in which the guide pulley is mounted between the plates so as to freely move in the axial direction is well known. See Official Gazette for Kokai Utility Model Application
In the case in which the guide pulley is mounted so as to freely move in the axial direction, even when the guide pulley does not match the center of the rear sprocket due to a failed adjustment, the guide pulley is shifted to the appropriate position by means of the chain's tension. Consequently, it becomes less likely that the chain will make noise or fall off.
In the traditional structure stated above, as an example, under conditions in which the rear derailleur is positioned at the outermost top position, when the front derailleur shifts gears from the top towards the middle, or from the middle towards the bottom, the tension of the chain suddenly decreases. At the same time, because the chain line which extends from the front chain wheel to the tension pulley becomes inclined inwards, the chain becomes subject to falling off inwards at the position close to the tension pulley. When the chain falls off from the tension pulley, the rider must stop the bicycle in order to re-derail the chain upon the tension pulley. Such re-detailing procedure is cumbersome. Especially in the midst of a race, such procedure will lead to a tremendous loss of time.
The purpose of the present invention is to achieve a return of the chain to the tension pulley more easily through pedaling actions when the chain falls off from the tension pulley.